The present invention relates to an image forming apparatus which enhances efficient transfer of a toner image to a transfer belt or similar transferring medium and a paper sheet or similar recording medium.
It is a common practice with a copier, printer or similar image forming apparatus to electrostatically form a latent image representative of a document image on a photoconductive element, develop the latent image to produce a toner image, and then electrostatically transfer the toner image to a recording medium such as a paper sheet. This kind of image forming process is executed in a one-sided front copy mode which transfers the toner image from the photoconductive element to the paper sheet or similar recording medium directly. Another image forming process available with an image forming apparatus uses a transfer belt or similar transferring medium located in close proximity to the photoconductive element. In this case, the toner image is once transferred to the transferring medium in matching relation to a record mode selected. Such a process is executed in a one-sided rear copy mode or in a two-sided copy mode. Specifically, in the one-sided rear copy mode, the toner image formed on the photoconductive element, i.e., a primary toner image is once transferred to the transfer belt by primary toner image transfer means, and then the toner image on the transfer belt, i.e., a secondary toner image is transferred to the rear surface of a paper sheet which is fed to the transfer belt. In the two-sided copy mode, a primary toner image formed on the photoconductive element first is transferred to the transfer belt as a secondary toner image, while a toner image is formed again on the photoconductive element. Subsequently, the secondary toner image on the transfer belt and the second toner image on the photoconductive element are transferred to a paper sheet at the same time by second toner image transfer means and first toner image transfer means, respectively. Such a scheme makes it needless to turn over the paper sheet in the two-sided copy mode and is disclosed in, for example, Japanese Patent Laid-Open Publication No. 35882/1988.
Any of the image forming processes described above is practicable with a single toner of particular color or a plurality of toners of different colors. When use is made of toners of different colors, a multicolor or a full-color image is achievable. Hence, by combining the toners of different colors and the different transfer processes, it is possible to implement:
(1) a monocolor or superposed-color one-sided front record mode; PA1 (2) monocolor or superposed-color one-sided rear record mode; and PA1 (3) monocolor or superposed-color two-sided record mode.
It is to be noted that the term "record mode" is equivalent to "copy mode", and that the term "superposed color" refers to both of multicolor and full-color.
The transfer means for executing the above-stated record modes may be implemented by roller transfer in addition to corona transfer and belt transfer. The roller transfer uses a bias roller made of conductive rubber or a dielectric roller produced by covering the surface of the bias roller with a dielectric film. Such a roller is pressed against the rear surface of a recording medium to form an electric field. The roller transfer causes a smaller amount of discharge to occur in an air gap than corona transfer or similar transfer, thereby suppressing the fall of the field effect and, therefore, enhancing transfer efficiency.
However, to execute any of the above-stated record modes, it is necessary to select a particular voltage application state for each of the transferring medium and secondary toner image transferring medium, so that images may be effectively transferred. Regarding the secondary toner image transfer means, among others, it is desirable to enhance the transfer quality by improving the transfer efficiency and eliminating defective images at the time of transfer, since the image on the transfer means of interest has been transferred from the photoconductive element. At the present stage of technologies, however, practical implementions or means capable of meeting the above requirement have not been reported.
After the image transfer, the paper sheet or similar recording medium is separated from the photoconductive element and then transported to a fixing device. At this instant, if the transfer characteristic does not have a sufficient margin, discharge is apt to occur at the time of separation to produce a defective toner image such as a locally omitted toner image or a blurred or dislocated toner image.
Further, the paper sheet is transported by the transport belt simply relying on the nip pressure acting between the photoconductive element and the transfer belt. In this condition, the leading edge of the paper sheet moved away from the transfer position is apt to lift away from the transfer belt or otherwise behave unstably, obstructing the stable entry of the paper sheet in the fixing device.